The present invention relates to electromagnetic valves. More particularly, the present invention pertains to an electromagnetic valve that is used as a displacement control valve in fluid machinery, especially in a variable displacement compressor employed in a vehicle air conditioner.
A typical automobile has a compressor that adjusts the temperature of a passenger compartment for making the ride comfortable for passengers. Often, this is a variable displacement compressor having a swash plate. The swash plate is tiltably supported on a drive shaft of the compressor. The inclination of the swash plate is changed in accordance with the difference between the pressure in a crank chamber and the suction pressure of the compressor. Rotation of the swash plate is converted into linear reciprocation of pistons.
Such a compressor includes an electromagnetic valve for controlling the displacement of the compressor. The valve adjusts the pressure in the crank chamber and the suction pressure for controlling the refrigeration performance of the compressor. Thus, an electromagnetic valve that accurately operates is needed for making the conditions of the passenger compartment comfortable for the passengers.
Typically, the valve includes a housing and an electromagnetic actuator that are secured to each other at the center of the valve. The valve housing includes a valve chamber. The valve chamber is located midway in a refrigerant passage and accommodates a valve body for opening and closing a valve hole. The valve also has a pressure sensing chamber. A pressure sensing member is housed in the pressure sensing chamber.
One end of the valve body is coupled to the actuator by a solenoid rod. The other end of the valve body is coupled to the pressure sensing member in the pressure sensing chamber by a pressure sensing rod. The pressure sensing member expands and contracts in accordance with the pressure in the pressure sensing chamber.
The actuator includes a fixed iron core, a steel plunger and a solenoid coil. The coil located radially outward of both the fixed core and the plunger. Electric current having a certain magnitude is supplied to the coil. The current produces a magnetic attractive force between the fixed core and the plunger in accordance with the current magnitude.
The attractive force is transmitted to the valve body by the solenoid rod. Also, when the pressure sensing member expands or contracts, the valve body receives the force from the sensing member through the pressure sensing rod. The opening area between the valve body and the valve hole is determined by the equilibrium of these forces. Accordingly, the flow rate of fluid in the passage of the valve is controlled.
When the pressure in the sensing chamber is lowered, the sensing member expands and is thus firmly held between an inner wall of the sensing chamber and the sensing rod. When the pressure in the sensing chamber increases, on the other hand, the sensing member contracts, which tends to separate the sensing member from the wall of the sensing chamber. In this state, the sensing member is supported only by an end of the sensing rod, which is inserted in a connector provided on the sensing member. A minute clearance exists between the sensing rod and the inner wall of the connector. Therefore, if the valve is vibrated, the sensing member tilts with respect to the sensing rod.
If the pressure in the sensing chamber is lowered again, the sensing member expands, which moves the sensing member toward the inner wall of the sensing chamber. In either case, the sensing member may be held between the sensing rod and the inner wall of the sensing chamber in a tilted state. As a result, the force of the sensing member is not accurately transmitted to the valve body through the sensing rod. This deteriorates the control accuracy of the electromagnetic valve.
The wall of the sensing member is made of a relatively thin material. Tilting of the sensing member may cause the sensing member's periphery to contact the inner wall of the sensing chamber. In this state, repeated expansions and contractions of the sensing member will wear the contacting part, which will shorten the life of the electromagnetic valve.